"We're taking measures we hope will prevent the groundwater communication with the creek in the location where we understand that to be happening," said Matt Lepore, director of Colorado's Oil and Gas Conservation Commission that regulates the industry.

State officials announced last week that benzene was detected in the creek for the first time since the spill was revealed on March 16. Diesel-like substances also have been detected in the creek, which flows into the Colorado River. The benzene detected last week was below the federal drinking water standard.

Absorbent booms have been laid across the creek, including near the head-gate for Parachute's reservoir, town administrator Bob Knight said.

Farmers and ranchers near Parachute use the reservoir water for irrigating crops. They rely on springs and other sources for drinking water, Knight said.

"Hopefully, by the time benzene gets to my cows, if it gets to my cows, it will be in low concentrations," said cattleman Rick Bumgardner, who grazes about 130 head, including calves, along the creek. "If it gets into the creek and gets to a toxic level, Williams will have to either haul me fresh, clean water or test my well and make sure it is fine."